The role of plasticizers during protein thermoplastic foaming

Protein thermoplastics, like Novatein, typically comprise a polymer, additives, and plasticizers. During foaming, the plasticizers performed different functions; triethylene glycol (TEG) affected the T-g of the polymer, while water altered the shear viscosity and urea the extensional viscosity. Water and urea also functioned as blowing agents; increasing urea increased the expansion ratio, while an increase in TEG and water decreased it. It was concluded that at this level of plasticization, phase separation occurred and that the plasticizers are most influential during late bubble growth and stabilization. Contrary to previous thought, increasing water had no effect on the T-g, but did lower the shear viscosity, while increasing TEG had the opposite effect. This enables the properties of protein thermoplastics to be tailored during development. During foaming, lower viscosity allows more time for gases to diffuse before the viscosity in the surrounding polymer increases, restricting bubble growth. At higher TEG content, the material is processed further above its T-g, slowing down stabilization and decreasing expansion. This study clarified the role of plasticizers during foaming and showed a decrease in random coils and beta-turns (measured by FT-IR) with increasing expansion ratio. Knowledge of these mechanisms enables tailoring properties of protein thermoplastics foams during development. (c) 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47781.